Two distinct genes, designated IL-1.alpha. and IL-1.beta., encode for interleukin-1 (IL-1) proteins IL-l.alpha. and IL-1.beta., respectively.
Interleukin IL-1.alpha. and IL-1.beta. are pleiotropic cytokines, and each, despite little sequence homology between them, exerts a variety of similar effects on different tissues and act on many human pathologies, and particularly on the immune response of the organism and on inflammatory processes. Both proteins have a molecular weight of about 17.5 kDa and are synthesized as a larger precursor molecule having a molecular weight of about 31 kDa.
IL-l proteins are potent inflammatory and pyrogenic cytokines that normally have beneficial effects, but that can also have extremely harmful effects on the organism. For example, IL-1 proteins participate in the pathogenesis of autoimmune pathologies, such as lupus erythematosus, and in particular, they are involved as mediators that provoke damage to tissues, as for example in rheumatoid arthritis.
Many of the biological effects of IL-1 are similar to those observed during sepsis. Recent studies demonstrated that the intravenous administration of IL-1 in a dosage range from 1 to 10 ng/kg gives rise to fever, sleepiness, anorexy, generalized myalgia, arthralgia and cephalea. Since IL-1 has pleiotropic biological activities, many of which negatively influence the organism, the powerful effects of IL-1 should be put under strict physiological control.
The synthesis of IL-proteins is inhibited by anti-inflammatory cytokines, prostaglandins and glucocorticoids and the existence of multiple levels of inhibition of IL-1 points to the necessity for strict control of this mediator. To date, IL-1 is the only cytokine for which an antagonist polypeptide for the receptor has been described; the third known component of the IL-1 family is the antagonist for the IL-1 receptor (IL-1ra).
All three components (IL-l.alpha., IL-1.beta., IL-1ra) recognize and bind to the same receptor on the cell surface (IL-1R); the binding of IL-1.alpha. and IL-1.beta. to IL-1R transmit a signal, whereas the binding of IL-1ra does not. There are two types of IL-1 receptors designated IL-1RI and IL-1RII. IL-1ra is a polypeptide which binds to IL-1RI, and also binds to IL-1RII with less affinity, without any agonistic activity.
IL-lra production is induced in different types of cells, including mononuclear phagocytes, polymorphonuclear cells (PMN) and fibroblasts, by IgG, cytokines and bacterial products. Until now, only two molecular forms of IL-lra have been identified and cloned:
1) secreted IL-lra (sIL-1ra) contains a classical leader sequence of 25 amino acids giving a mature protein of 152 amino acids; PA1 2) intracellular IL-lra (icIL-1ra) lacks a leader sequence and it is predicted that this protein remains intracellular. sIL-1ra and icIL-1ra are generated from the same gene.
The human genes coding for IL-1.alpha. and IL-1.beta. and IL-1ra belongs to the same gene family and are clusterized on chromosome 2 mapping to the q12-q21 region (Eisenberg et al., Proc. Natl. Acad. Sci. USA 88:5232, 1991). The human IL-1ra gene has been partially sequenced and characterized. As shown in FIG. 5, the human IL-1ra gene contains in its 3' region, three exons coding for the C-terminal common part of the three isoforms of IL-1ra (exons 2, 3 and 4 in FIG. 5) and in its 5' region, three exons which can be differentially spliced to generate the three isoforms of the antagonist (exons ic1, ic2 and s1 in FIG. 5). The 3' part of the gene between the promoter of the secreted isoform and the last exon of the gene, as well the region upstream the first intracellular specific exon, have been sequenced (bold line in FIG. 5) (Butcher et al., J. Immunol.153:701, 1994; Lennard et al., Cytokine 4:83, 1992). On the other hand, the part of the gene between the promoter region of sIL-ra and first exon of icIL-1ra was unknown (thin line in FIG. 5).
Transcripts of icIL-1ra1 originate from an alternative starting site and from the splicing of a first alternative exon into an internal splice acceptor site located in the first exon of sIL-lra. The predicted proteins are thus identical except in their NH.sub.2 ends, where the first 21 amino acids of sIL-1ra are substituted by four amino acids in icIL-Ira. The expression of transcripts encoding sIL-lra and icIL-lra is regulated differently, and the biological significance of icIL-lra is still unclear.
Considering that IL-1 is involved in pathogenesis of many diseases, it is evident that there is a need for medicaments which are useful in limiting the harmful effects of IL-1.
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